This invention relates to the production of benzene from a stabilized hydrocarbon fraction comprising alkylaromatic hydrocarbons, unsaturated hydrocarbons and sulfur compounds.
A known method for producing benzene from a stabilized hydrocarbons fraction, i.e. a fraction made free of the components of lower stability (diolefins and acetylenics), consists of subjecting this fraction to catalytic hydrodesulfuration, followed with hydrodealkylation. The unconverted C.sub.7 and C.sub.8 hydrocarbons may be recycled. When the hydrocarbon charge has a low sulfur content, for example less than 100 ppm by weight (b.w.) no serious difficulty is encountered. Conversely, when the hydrocarbon charge has a higher sulfur content, for example more than 100 ppm b.w., the catalytic hydrodesulfuration which converts the organic sulfur compounds mainly to H.sub.2 S must be followed with cooling and condensation of the hydrocarbon fraction, so as to separate it from a gas phase containing hydrogen sulfide. As a matter of fact, the presence of H.sub.2 S in substantial amounts in the hydrodealkylation step results in the formation of thiophene which cannot be separated easily from benzene. The resultant benzene does not comply with the specifications of industry (a maximum of 1 ppm b.w. of thiophene).
The fractionation destined to eliminate H.sub.2 S before hydrodealkylation is costly as concerns investment and energy since the hydrodesulfuration effluent must be cooled and then reheated to the elevated hydrodealkylation temperature.
According to U.S. Pat. No. 3,222,410 and U.S. Pat. No. 3,310,592, stabilized gasoline is subjected to hydrodealkylation followed with hydrodesulfuration of benzene. This process is not adapted to extended hydrodealkylation periods, since coal-like particles separate, and the benzene yield is poor, as unconverted toluene and xylenes cannot be recycled to the hydrodealkylation step, due to their content of thiophane and other sulfur compounds.